WO2001085017A1

WO2001085017A1 - Method and apparatus for determining size of an optic nerve head

Info

Publication number: WO2001085017A1
Application number: PCT/GB2001/002074
Authority: WO
Inventors: Timothy Richard Holbrook Dawson
Original assignee: Marcher Enterprises Limited
Priority date: 2000-05-12
Filing date: 2001-05-11
Publication date: 2001-11-15
Also published as: GB0011457D0; AU2001256485A1

Abstract

The size of an optic nerve head (20) of a test eye (26) is determined with the aid of an optical imaging arrangement (28, 32) for focussing an image of the optic nerve head on a displaceable imaging means (30). A reference position is established for the imaging means, the reference position being a position of focus for an emmetropic eye. A value of radius of curvature appropriate to a cornea of the test eye (26) is established. Displacement of the imaging means (30) is determined from the reference position to a position in which the optic nerve head (20) is in focus on the imaging means. Data are established relating magnification of images and displacement of the imaging means (30) from the reference position to positions of focus on the imaging means, as a function of corneal radius of curvature. The displacement of the imaging means (30) from the reference position for the test eye (26) is applied to the data and a value is derived of the magnification in accordance with the appropriate corneal radius of curvature.

Description

METHOD AND APPARATUS FOR DETERMINING SIZE OF AN OPTIC NERVE HEAD

This invention relates to a method and apparatus for determining size of an optic nerve head in an eye.

In patients with glaucoma, changes in the shape of the optic nerve head or disc occur, and can be detected, up to two years or more before there is perceptible loss of visual field. A typical change which occurs is for the central portion of the optic nerve head (disc) to enlarge and deepen to form a cup. Progress has hitherto generally been monitored by determining the ratio of the diameter of the cup to the diameter of the disc, in vertical and horizontal directions.

Recent work has suggested that it is not the size of the cup that is important, but rather the size of a remaining annular region surrounding the cup and which is referred to as the neuro-retinal rim.

Nerve fibres from the retina of the eye pass through the rim to the brain via the optic nerve. In a small disc the nerve fibres are more closely packed than in a large disc, each normal individual person having approximately the same number of nerve fibres (about 1.2 million) in a healthy eye.

Accordingly, loss of a given amount of absolute rim area in a small disc represents a greater loss of nerve fibres than in a large disc. A particular cup to disc size ratio (for example 0.6) in a large disc, such as of 1.5mm diameter or more, may result in an adequate absolute area of rim. However the same cup to disc size ratio in a small disc, such as of 1mm diameter or less, results in a smaller area of rim, which may be inadequate. Consequently, the smaller the disc, the lower will be a critical size ratio of cup to disc.

It would therefore be desirable, when diagnosing and monitoring glaucoma, to be able to determine not only the size ratio of cup to disc, or rim to disc, but also the absolute size of the rim area.

Furthermore, a further indication of glaucoma is a difference between the discs in left and right eyes of an individual (referred to as asymmetry) . Since refraction of the left and right eyes may differ, as a result of difference in corneal curvatures, the discs cannot readily be compared unless their absolute sizes are known.

It is an object of the present invention to overcome or minimise these problems.

According to one aspect of the present invention there is provided a method of determining size of an optic nerve head of a test eye, comprising: providing an optical imaging arrangement for focussing an image of the optic nerve head on a displaceable imaging means; establishing a reference position for the imaging means, the reference position being a position of focus for an emmetropic eye; establishing a value of radius of curvature appropriate to a cornea of the test eye; determining displacement of the imaging means from the reference position to a position in which the optic nerve head is in focus on the imaging means; establishing data relating magnification of images and displacement of the imaging means from the reference position to positions of focus on the imaging means, as a function of corneal radius of curvature; and applying to the data the displacement of the imaging means from the reference position for the test eye and deriving therefrom a value of the magnification in accordance with the appropriate corneal radius of curvature .

From another aspect of the present invention there is provided apparatus for determining size of an optic nerve head of a test eye, comprising: an optical imaging arrangement for focussing an image of the optic nerve head on a displaceable imaging means; means to establish a reference position for the imaging means, the reference position being a position of focus for an emmetropic eye; means to establish a value of radius of curvature appropriate to a cornea of the test eye; means to determine displacement of the imaging means from the reference position to a position in which the optic nerve head is in focus on the imaging means; means to establish data relating magnification of images and displacement of the imaging means from the reference position to positions of focus on the imaging means, as a function of corneal radius of curvature; and means to apply to the data the displacement of the imaging means from the reference position for the test eye and to derive therefrom a value of the magnification in accordance with the appropriate corneal radius of curvature .

The size of the optic nerve head is determined from the value of the magnification and measurement of size of the image thereof on the imaging means .

The data relating magnification of images and displacement of the imaging means from the reference position may be established by modelling with a model eye, for example by means of computer modelling. The data relating magnification to displacement may provide a substantially linear graphical relationship between image magnification and displacement of the imaging means from the reference position to positions of focus on the imaging means, for each selected value of corneal radius of curvature .

The imaging means may comprise a charge coupled device (CCD) arrangement which may be employed in a video camera or the like, and associated display means.

The imaging means may alternatively comprise means to permit immediate viewing by a person.

The value of the radius of curvature appropriate to the cornea of the test eye may be obtained by means of a keratometer. Alternatively, a statistical average value of radius of curvature may be employed.

The expression "emmetropic eye" as used herein refers to an eye providing substantially perfect distance vision when parallel rays of light, such as from a distant object, are brought to focus on the retina of the eye without correction. Similarly, rays of light from the optic nerve head emerge from the eye and approach the imaging means in parallel bundles, as though they had originated from infinity.

The optical imaging arrangement may involve a method for focussing images, which method comprises the steps of: providing an optical system including at least first and second apertures, the apertures being provided substantially symmetrically about an optical axis of the system so as to provide separate sub-images formed by the at least first and second apertures; sequentially analysing the separate sub-images in a manner such that the sub-images are superimposed to form a stable image when the image is in focus; and adjusting the optical system to generate a stable image formed by the sequential sub-images.

The optical imaging arrangement may also or alternatively involve an apparatus for focussing images, which apparatus comprises an optical system including: at least first and second apertures, the apertures being positioned substantially symmetrically about an optical axis of the system so as to provide separate sub-images formed by the at least first and second apertures; means for analysing the separate sub-images sequentially in a manner such that the sub-images are superimposed to form a stable image when the image is in focus; and means for adjusting the optical system so as to permit focussing of an image.

The separate sub-images may be displayed for manual analysis and/or may be analysed by computer.

The separate sub-images may be analysed sequentially by opening and closing the apertures in sequence.

Alternatively, the separate sub-images may be generated simultaneously and may be analysed sequentially in the same space such that the sub-images are superimposed to form a stable image when the image is in focus.

The image may be focussed for immediate viewing by a person or may be generated by an imaging means, such as a CCD device, for display.

The sub-images may be displayed sequentially at a rate in the range of about three to fifteen or more sub-images per second, preferably either in the range of about three to ten sub-images per second or in the range of about ten to fifteen sub-images per second. The optical system may be adjusted, for example, by adjusting the position from which the image is viewed.

For a better understanding of the invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a simplified front view representation of an optic nerve head (or disc) of an eye;

Figure 2 is a diagrammatic illustration of an embodiment of an apparatus according to the present invention;

Figure 3 is a representation of a model eye used in the method and apparatus of the present invention;

Figure 4 is a graph showing the relationship between image magnification and displacement of a charge coupled device (CCD) imaging means as a function of corneal radius of curvature, derived by computer modelling;

Figure 5 is a diagrammatic illustration of one embodiment of an image focussing apparatus for use in the method and apparatus of the invention;

Figure 6 is a diagrammatic illustration corresponding to Figure 5, with the image being focussed beyond an imaging means ;

Figure 7 is a diagrammatic illustration corresponding to Figure 5 , with the image being focussed before an imaging means ;

Figure 8 is a diagrammatic illustration of another embodiment of an image focussing apparatus for use in the method and apparatus of the invention, the image being in focus;

Figure 9 is a diagrammatic illustration corresponding to Figure 8, with the image being focussed beyond an imaging means ;

Figure 10 is a diagrammatic illustration corresponding to Figure 8, with the image being focussed before an imaging means;

Figure 11 shows the image focussing apparatus of Figures 8 to 10 incorporating a computer for processing captured images ; and

Figure 12 illustrates one manner in which focussing of the image can be adjusted.

Referring to Figure 1, an eye 26 has an optic nerve head 20, sometimes referred to as a disc. The optic nerve head (disc) 20 has a central region 22 and a surrounding peripheral region 24.

In patients with glaucoma, changes in the shape of the optic nerve head (disc) occur, and can be detected, up to two years or more before there is perceptible loss of visual field. A typical change which occurs is for the central region 22 of the disc 20 to enlarge and deepen to form a cup, leaving a doughnut-shaped surrounding peripheral region 24. The size of this peripheral region 24, which is known as the neuro-retinal rim, is important and consequently it is required to be able to determine the absolute size of the disc 20 and hence of the peripheral region 24.

Referring to Figure 2, the eye 26 to be tested incorporates the optic nerve head (disc) 20 of Figure 1. Using an optical imaging arrangement 28, 32, such as will be described hereinafter with reference to Figures 5 to 12, an image of the disc 20 is able to be focussed on an imaging means, suitably comprising a charge coupled device (CCD) arrangement 30. The CCD arrangement 30 may be employed in a video camera 32, or the like, and connected to a well known form of image display means, such as a display screen.

The CCD arrangement 30 is arranged to be displaceable in the directions of arrows 34, such displacement being suitably effected by a motor driving means, such as a stepper motor.

The eye 26 incorporates a lens 36 and a cornea 38. The cornea 38 has a particular radius of curvature, which is required to be known, at least approximately, and is determined using a well known keratometer device. If the radius of curvature cannot be determined at the time of test of the eye 26, a value obtained during an earlier test of the eye may be used, or a statistical average value used.

A reference position is established for the CCD arrangement 30. This is a position of focus for an emmetropic eye, which is an eye providing substantially perfect distance vision. With such an eye, parallel rays of light, such as from a distant object, are brought to focus on the retina of the eye without any correction being required. Rays of light from the optic nerve head (disc) emerge from the eye and approach the CCD arrangement 30 in parallel, as though they had originated from infinity.

To establish the reference position, the camera 32 is focussed on a distant object, such as an object at a distance of about 1 metre. When the distant object is in focus on the CCD arrangement 30, this is the reference position of the CCD arrangement 30 and can be preset in a workshop .

In use of the apparatus, an optical sensor switch (not shown) can be provided to detect when the CCD arrangement 30 is in the reference position and can be arranged to set a visible "flag" on a computer monitor.

The camera 32 is then put into an "alignment" mode in which auxiliary lenses are introduced, such as by means of a stepper motor, into the optical path inside the camera such that it can focus on an object at a distance of about 50mm in front of the camera. In this mode, the entire camera 32 is moved towards the eye 26 under test until an image of the pupil of the eye 26 is in sharp focus on the CCD arrangement 30. In this way a set and repeatable distance is achieved between the eye 26 under test and the camera 32. The auxiliary lenses are then moved out of the optical path and the camera is thus now focussed on "infinity". An image of the optic nerve head (disc) 20 is captured on the CCD arrangement 30. If the eye 26 under test is emmetropic, such that it needs no correction for distant vision, the image of the optic nerve head (disc) 20 will be in sharp focus on the CCD arrangement 30. If a sharply focussed image is not obtained, the CCD arrangement 30 is displaced from its reference position, in one of the directions of the arrows 34, until sharp focussing is obtained. The displacement from the reference position is measured. If the corneal curvature of the eye is regarded as fixed, or its value known, the displacement of the CCD arrangement 30 to achieve focus is determined only by the axial length of the eye, that is the distance between the front of the cornea and the retina (ignoring the effects of variations in the crystalline lens) .

The displacement of the CCD arrangement 30 from the reference position is used to determine the magnification of the image captured on the CCD arrangement 30, using data determined from the following procedure.

A computerised model of the arrangement of Figure 2 was made, including a computer modelled emmetropic eye 40 as shown in Figure 3. The model eye 40 was arranged to have a particular average value of radius of curvature (7.8mm) for the cornea 42, a particular radius of curvature (11.514mm) for the front 44 of the crystalline lens 46, a particular radius of curvature (-6.0mm) for the rear 48 of the crystalline lens 46, and a particular radius of curvature (-10.0mm) for the retina 50. The refractive index for each of the cornea 42, the aqueous humour 52 and the vitreous humour 54 was selected as 1.333. The refractive index of the lens 46 was selected as 1.450.

With the emmetropic model eye 40, parallel rays of light 56 from a distant object are brought to focus on the retina 50 without correction.

A computerised model of the arrangement of Figure 2, without the eye 26 present, was checked at infinity focus to determine the emmetropic focus condition. The emmetropic model eye 40 was then added to this model to give a complete imaging path and the conditions were checked again to ensure that there was no change in the focus position.

In order to test the model at set repeatable steps in the focus movement, the model was reversed so that the CCD arrangement 30 effectively became the "object" and the retina 50 the "image". The computer modelling was set to optimise focussing automatically by adjusting the distance between the rear surface 48 of the lens 46 and the retina 50. With the radius of curvature of the cornea 42 of the model eye 40 set to the predetermined average value (7.8mm), the CCD arrangement 30 was effectively displaced in fixed steps of 1mm (range -8mm to +10mm) from the emmetropic reference position and at each step the distance between the rear surface 48 of the lens 46 and the retina 50 was read from the computer software, as was also the magnification of the image. The magnification obtained from this reversed model was then inverted to provide the correct magnification for the actual system.

It was found that the magnification plotted against CCD arrangement displacement showed a straight line function.

The modelling process was repeated for different values of radius of curvature for the cornea 42 of the model eye 40, covering a range of values from about 7.0mm to about 8.8mm. As shown in Figure 4, a series of almost parallel straight lines resulted for the magnification plotted against CCD arrangement displacement. In Figure 4 the line 58 for a radius of curvature of 7.0mm is shown and also the line 60 for a radius of curvature of 8.8mm. The corresponding lines for radii of curvature between 7.0mm and 8.8mm lie between these lines 58 and 60.

The family of straight lines follow a relationship of y = mx + c, where y represents the magnification and x the CCD arrangement displacement (mm) . The slope m of the lines decreases very slightly from a value of 0.0604, at a radius of curvature of 7.0mm, to a value of 0.0583, at a radius of curvature of 8.8mm. The value of c decreases progressively from 1.4317, at a radius of curvature of 7.0mm, to 1.2597 at a radius of curvature of 8.8mm.

The value of the displacement of the CCD arrangement 30 from the reference position for the actual test eye 26 is used with the data of Figure 4 to derive a value for the magnification of the image of the optic nerve head (disc) 20 captured on the CCD arrangement 30, having regard to the radius of curvature of the cornea of the test eye 26.

The size of the image of the optic nerve head (disc) 20 captured on the CCD arrangement 30 is measured and used with the magnification data to calculate the size of the optic nerve head (disc) 20 and/or parts thereof, such as the cup-shaped central region 22 or the peripheral region 24 (the neuro-retinal rim) .

A method and apparatus for focussing images, for use in the optical arrangement 28 and camera 32 in the method and apparatus of the present invention, is disclosed in WO 99/59014.

Embodiments of such a method and apparatus for focussing images will now be described with reference to Figures 5 to 12.

The image focussing apparatus shown in Figures 5, 6 and 7 includes an opaque plate 1 which is provided with two apertures 3 which are positioned symmetrically to either side of an optical axis 5. The plate 1 is provided with shutter means 7 which is adapted to cover one or the other of the apertures 3 alternately. Thus, the visual effect of the shutter and aperture arrangement is that the shutter oscillates between the two apertures.

If desired more than two apertures 3 can be provided positioned substantially symmetrically about the optical axis 5, the shutters 7 being opened and closed in sequence.

In a focussed optical system as illustrated in Figure 5, rays forming an image converge to meet at the focal plane which coincides with an imaging means 9, for example a CCD device employed in a video camera or the like. When the imaging means 9 does not coincide with the focal plane 11 of the optical system, that is when the imaging means is axially offset relative to the focal plane, the rays no longer meet at the plane of the imaging means and the resulting image is blurred. This can be seen from Figure 6 where the focal plane 11 is beyond the plane of the imaging means 9 and from Figure 7 where the focal plane 11 is before the plane of the imaging means 9.

Where the aperture is small, the depth of field is relatively large and blurring of the image is not readily apparent until the offset between the focal plane 11 and the plane of the imaging means 9 is relatively substantial. The lack of apparent blurring renders focussing of the image by examination relatively difficult.

Nevertheless, each aperture 3 gives rise to separate sub- images and the sub-images from the two apertures will only coincide at the focal plane 11. When the imaging means 9 is offset from the focal plane, two (slightly blurred) sub- images are formed which are offset laterally relative to each other in the plane of the imaging means 9.

When the shutter means 7 is operated to cover one or other of the apertures 3 alternately, for example the shutter means operating at a rate of about 3 to 15 cycles per second, first one of the laterally offset sub-images is formed and then the other. Thus, an out of focus image oscillates laterally, the image appearing to jump from side to side, with a period corresponding to the cycle time of the shutter means, while a focussed image remains stationary.

It has been found that oscillation of an out of focus image provides a particularly sensitive method for focussing such an image . Focussing of the image can be accomplished in any conventional manner, preferably by adjusting the position of the imaging means 9 along the optical axis, that is by adjusting the position from which the image is viewed. This is accomplished, for example as illustrated in Figure 12, by mounting the imaging means on a rack assembly 16, the rack being driven by a pinion 13 on which there is provided an adjusting knob 15 for manual operation and/or which is connected to a stepper motor for computer control. As an alternative (not illustrated) , the lens and plate 1 can be moved along the optical axis, with or without the imaging means 9. Nevertheless, it has been found focussing by moving the imaging means 9 alone to be most satisfactory because the change in magnification is least.

The shutter means 7 may comprise any suitable mechanism such as those described in GB-A-2 283 828. That is, the shutter means 7 could comprise one or more mechanical shutters operable to close the apertures 3 in sequence. Alternatively, the shutter means 7 could comprise a pair of polarising filters for each aperture 3 and arranged such that the planes of polarisation are mutually perpendicular. As a further alternative, the shutter means 7 could comprise a liquid crystal shutter arrangement incorporating two liquid crystal shutters controllable to allow light to pass through each shutter in turn.

The shutters need not be provided in a plate of any substantial dimensions, the need being only to provide a means for defining apertures of the required size.

The image focussing apparatus can be used, for example with a wide range of optical instruments and apparatus including ophthalmic apparatus and instruments and microscopes, especially electron microscopes. The image focussing apparatus described in relation to Figures 5 to 7 is relatively simple to construct and operate. However, it suffers the disadvantage that the two sub-images are analysed or displayed alternately and it is desirable to eliminate the possibility of the image changing, for example as a result of a subject moving, between alternate sub-images.

The image focussing apparatus shown in Figures 8, 9 and 10 enables two sub-images to be analysed or displayed simultaneously. Instead of occupying the same space on the imaging plane as in the embodiment of Figures 5 to 7, the sub-images are optically directed to different areas of the imaging plane thus eliminating the need for a shutter.

The image focussing apparatus shown in Figures 8, 9 and 10 includes an opaque plate 101 which is provided with two apertures 103 which are positioned symmetrically to either side of an optical axis 105.

In a focussed optical system as illustrated in Figure 8, rays forming two sub-images converge to meet at the focal plane which coincides with an imaging means 107, for example a CCD device employed in a video camera or the like. The two sub-images falling on the CCD device can be processed by a computer or the like to display two focussed sub-images in registry with each other.

When the imaging means 107 does not coincide with the focal plane 109 of the optical system, that is when the imaging means is axially offset relative to the focal plane, the rays no longer meet at the plane of the imaging means and the resulting sub-images are blurred. This can be seen from Figure 9 where the focal plane 109 is beyond the plane of the imaging means 107 and the sub-images are further apart at the imaging means 107 than in Figure 8 and from Figure 10 where the focal plane 109 is before the plane of the imaging means 107 and the sub-images are closer together at the imaging means 107 than in Figure 8.

As with the embodiment of Figures 5 to 7, where the aperture is small, the depth of field is relatively large and blurring of the sub-images is not readily apparent until the offset between the focal plane 109 and the plane of the imaging means 107 is relatively substantial.

Nevertheless, each aperture 103 gives rise to separate sub- images and the images from the two apertures will only coincide at the focal plane 109 otherwise they will be laterally offset relative to each other. That is, when the imaging means 107 is offset from the focal plane, two (slightly blurred) sub-images are formed which are offset relative to each other in the plane of the imaging means 107.

In this case the two sub-images are gathered simultaneously and can be analysed alternately by computer or displayed alternately on a display such as a computer screen, albeit without the need for a shutter. This has the advantage of eliminating patient or object movement between the two sub- images .

The overall image is in focus when the two sub-images correspond each other without any lateral movement between the sub-images when they are displayed alternately on a computer screen. As with the embodiment of Figure 5, we have found it is acceptable to alternate the sub-images at a rate of about 2 to 10 (preferably about 3 to 4) times per second and an out-of-focus image oscillates laterally, the image appearing to jump from side to side, with a period corresponding to the rate of change of the sub-images, while a focussed image remains stationary. However, it would also be possible to alternate the sub-images at the video frame rate (25 to 30 times per second) . As with the embodiment of Figure 5 to 7, focussing of the image can be accomplished in any conventional manner, such as that shown in Figure 12. Of course, fresh sub-images are gathered as the image is focussed. However, as an alternative in the case where focussing is accomplished by means of a stepper motor as described above in relation to Figure 12, the sub-images can be processed by computer to determine whether or not they are in focus and the focussing can also be controlled by the computer and adjusted until the sub-images correspond each other without any lateral movement therebetween. However, supplemental manual control would enable manual override of the computer if required.

Figure 11 additionally illustrates camera means 111 incorporating the imaging means 107 and a computer 113 for processing captured images and displaying such images alternately on a display 115.

Claims

1. A method of determining size of an optic nerve head (20) of a test eye (26) , comprising: providing an optical imaging arrangement (28, 32) for focussing an image of the optic nerve head on a displaceable imaging means (30) ; establishing a reference position for the imaging means, the reference position being a position of focus for an emmetropic eye; establishing a value of radius of curvature appropriate to a cornea of the test eye (26) ; determining displacement of the imaging means (30) from the reference position to a position in which the optic nerve head (20) is in focus on the imaging means; establishing data relating magnification of images and displacement of the imaging means (30) from the reference position to positions of focus on the imaging means, as a function of corneal radius of curvature; and applying to the data the displacement of the imaging means (30) from the reference position for the test eye (26) and deriving therefrom a value of the magnification in accordance with the appropriate corneal radius of curvature .

2. A method according to claim 1 and including the step of determining the size of the optic nerve head (20) from the value of the magnification and measurement of size of the image thereof on the imaging means (30) .

3. A method according to claim 1 or 2, wherein the data relating magnification of images and displacement of the imaging means (30) from the reference position are established by modelling with a model eye.

4. A method according to claim 3, wherein the modelling is effected by means of computer modelling.

5. A method according to any preceding claim, wherein the data relating magnification to displacement provides a substantially linear graphical relationship between image magnification and displacement of the imaging means (30) from the reference position to positions of focus on the imaging means, for each selected value of corneal radius of curvature .

6. A method according to any preceding claim, wherein the imaging means (30) comprises a charge coupled device (CCD) arrangement and associated display means.

7. A method according to claim 6, wherein the charge coupled device (CCD) arrangement is employed in a video camera (32) .

8. A method according to any one of claims 1 to 5, wherein the imaging means (30) comprises means to permit immediate viewing by a person.

9. A method according to any preceding claim, wherein the value of the radius of curvature appropriate to the cornea of the test eye (26) is obtained by means of a keratometer.

10. A method according to any one of claims 1 to 8, wherein a statistical average value of radius of curvature is employed.

11. A method according to any preceding claim, wherein the optical imaging arrangement (28, 32) involves a method for focussing images, which method comprises the steps of: providing an optical system including at least first and second apertures (3, 103), the apertures being provided substantially symmetrically about an optical axis (5, 105) of the system so as to provide separate sub-images formed by the at least first and second apertures; sequentially analysing the separate sub-images in a manner such that the sub-images are superimposed to form a stable image when the image is in focus; and adjusting the optical system to generate a stable image formed by the sequential sub-images.

12. An apparatus for determining size of an optic nerve head (20) of a test eye (26), comprising: an optical imaging arrangement (28, 32) for focussing an image of the optic nerve head on a displaceable imaging means (30) ; means to establish a reference position for the imaging means, the reference position being a position of focus for an emmetropic eye; means to establish a value of radius of curvature appropriate to a cornea of the test eye (26) ; means to determine displacement of the imaging means (30) from the reference position to a position in which the optic nerve head (20) is in focus on the imaging means; means to establish data relating magnification of images and displacement of the imaging means (30) from the reference position to positions of focus on the imaging means, as a function of corneal radius of curvature; and means to apply to the data the displacement of the imaging means (30) from the reference position for the test eye (26) and to derive therefrom a value of the magnification in accordance with the appropriate corneal radius of curvature .

13. An apparatus as claimed in claim 12 and including means for determining the size of the optic nerve head (20) from the value of the magnification and measurement of size of the image thereof on the imaging means (30) .

14. An apparatus as claimed in claim 12 or 13, wherein the data relating magnification of images and displacement of the imaging means (30) from the reference position are established by modelling with a model eye.

15. An apparatus as claimed in claim 14, wherein the modelling is effected by means of computer modelling.

16. An apparatus as claimed in any one of claims 12 to 15, wherein the data relating magnification to displacement provides a substantially linear graphical relationship between image magnification and displacement of the imaging means (30) from the reference position to positions of focus on the imaging means, for each selected value of corneal radius of curvature .

17. An apparatus as claimed in any one of claims 12 to 16, wherein the imaging means (30) comprises a charge coupled device (CCD) arrangement and associated display means.

18. An apparatus as claimed in claim 17, wherein the charge coupled device (CCD) arrangement is employed in a video camera (32) .

19. An apparatus as claimed in any one of claims 12 to 16, wherein the imaging means (30) comprises means to permit immediate viewing by a person.

20. An apparatus as claimed in any one of claims 12 to 19, wherein the value of the radius of curvature appropriate to the cornea of the test eye (26) is obtained by means of a keratometer.

21. An apparatus as claimed in any one of claims 12 to 19, wherein a statistical average value of radius of curvature is employed.

22. An apparatus as claimed in any one of claims 12 to 21, wherein the optical imaging arrangement (28, 32) involves an apparatus for focussing images, which apparatus comprises an optical system including: at least first and second apertures (3) , the apertures being positioned substantially symmetrically about an optical axis (5) of the system so as to provide separate sub-images formed by the at least first and second apertures ; means for analysing the separate sub-images sequentially in a manner such that the sub-images are superimposed to form a stable image when the image is in focus; and means for adjusting the optical system so as to permit focussing of an image.